In many nickel-catalyzed, high-temperature processes such as methanation, steam reforming, steam gasification of naphthas and the like, a long standing problem has been that of maintaining activity of the catalyst over extended periods of time. In such processes, deactivation can occur over process temperature ranges of about 500.degree.-2500.degree. F. as a result of sulfur poisoning, to which the nickel-alumina catalysts are extremely susceptible. At temperatures above about 1100.degree.-1200.degree. F., deactivation can also occur as a result of nickel crystallite growth, with resultant reduction in active surface area.
Nickel-alumina catalysts for use in conventional methanation processes must, for maximum effectiveness, contain relatively large proportions of nickel, generally in excess of about 20 wt.% and usually between about 30 and 60 percent by weight as NiO. This requirement places certain limitations on practical methods for manufacturing such catalysts. Conventional impregnation techniques of preformed alumina supports with nickel salt solutions require multiple impregnations with intervening calcinations, and generally result in catalysts having an unacceptably low pore volume. Comulling the powdered alumina base with sufficient concentrated nickel salt solution to provide an extrudable mixture generally does not give the desired nickel content and may also adversely affect porosity.
To avoid the foregoing difficulties, the art has been forced to resort to two principal catalyst manufacturing methods. One involves simply comulling the powdered alumina base with finely powdered nickel oxide, and then tableting or extruding the mixture. The resulting catalysts display poor activity and thermal stability due to poor nickel dispersion and minimal interaction with the alumina component. Somewhat more effective catalysts are prepared commercially by aqueous coprecipitation methods, in which aluminum hydroxide and nickel hydroxide are coprecipitated from an aqueous solution of salts of the two metals upon addition thereto of a base such as ammonium hydroxide. The most effective of such coprecipitated catalysts which I have found described in the literature is that of U.S. Pat. No. 3,320,182 to Taylor et al, which discloses a coprecipitated nickel-alumina catalyst, employing as the precipitant, ammonium bicarbonate. However, as will be shown hereinafter, this catalyst, as well as other coprecipitated species, are very definitely inferior in thermal stability, compared to the catalysts of this invention.
In brief summary, the catalysts of this invention are prepared by precipitation of nickel hydroxide in a slurry consisting of a powdered alumina hydrate dispersed in an aqueous solution of an ammino complex of a nickel salt. After thoroughly mixing the slurry at a temperature sufficiently low to avoid decomposition of the ammino complex, heat is applied to the mixture with vigorous agitation to effect a gradual decomposition of the ammino complex with resultant liberation of ammonia. This results in a very gradual precipitation of nickel hydroxide within the fine pore structure of the alumina hydrate particles and upon their external surfaces. The rate of precipitation is controlled by the heat input. Ordinarily, the slurry is brought to boiling to expel excess ammonia. When the precipitation is completed, the solids are collected as by filtration, washed, dried and subjected to a calcination which is believed to bring about formation of substantial quantities of nickel aluminate, NiAl.sub.2 O.sub.4. The calcined powder can then be mulled and extruded, or tableted to form the desired catalyst shape. No added binder is ordinarily required. The tableted or extruded material is than again calcined in air at, e.g., 900.degree. F.
A critical feature in the foregoing preparation is believed to reside in the use of an alumina hydrate, preferably boehmite, in the aqueous slurry from which nickel hydroxide is precipitated. The hydrated aluminas are believed to foster the combination therewith of nickel hydroxide, resulting ultimately in the formation of a substantial moiety of nickel aluminate. While the remarkable thermal stability of the present catalysts cannot be accounted for with certainty, it is believed that the active species of nickel in the finished catalyst is in a metallic form which can gradually become poisoned by sulfur or reduced in activity by crystallite growth. The nickel aluminate component is very difficultly reducible, and does not per se contribute any significant catalytic activity. However, it is believed that the nickel aluminate, or some such similar compound, acts somewhat as a "reservoir" which, upon gradual reduction during use in the presence of hydrogen and/or carbon monoxidecontaining gases, continually generates fresh metallic nickel, thus accounting for the remarkable activity maintenance of the catalysts.
In U.S. Pat. No. 3,988,263 to Hansford, another very stable nickel-alumina methanation catalyst is disclosed which is prepared by a unique coprecipitation method, and also contains nickel aluminate crystallites. However, electron microscopy and high resolution electron diffraction examination has detected definite physical differences between the present catalysts and those of the patent; the catalysts of the patent are found to be relatively more heterogeneous in nature, some areas being almost free of detectable Al.sub.2 O.sub.3 and NiAl.sub.2 O.sub.4 while in the present catalysts all sampled areas appear more homogeneous, with 20-70 A NiAl.sub.2 O.sub.4 particles typically surrounding the larger NiO crystals.
Also, a characteristic NiAl.sub.2 O.sub.4 diffraction spacing of medium intensity at d=4.56 is found in the catalysts of the patent but in the present catalysts all of the detectable NiAl.sub.2 O.sub.4 d values are below 3. This indicates a difference in crystalline orientation of NiAl.sub.2 O.sub.4 in the respective catalysts, namely presence of the (111) plane in the NiAl.sub.2 O.sub.4 crystallites of the patented catalysts, and its absence from the crystallites of the present cayalysts. Absence of the (111) plane usually tends to lead to greater reactivity such as, in the present case, ease of reduction to metallic Ni. It is clear however that NiAl.sub.2 O.sub.4 crystallites are present in both catalysts since they both produce the major standard NiAl.sub.2 O.sub.4 diffraction spacings at d=2.43.+-.0.01, d=2.01.+-.0.01 and d=1.42.+-.0.01, but no NiAl.sub.2 O.sub.4 spacing about d=3. All of these lines however are stronger in the case of the present catalysts than those of the patented catalysts.